drm/rockchip: plane_state->fb iff plane_state->crtc

Checking both is one too much, so wrap a WARN_ON around it to stope
the copypasta.
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Sandy Huang <hjc@rock-chips.com>
Cc: "Heiko Stübner" <heiko@sntech.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-rockchip@lists.infradead.org
Link: https://patchwork.freedesktop.org/patch/msgid/20191213172612.1514842-6-daniel.vetter@ffwll.ch

drivers/gpu/drm/rockchip/rockchip_drm_vop.c

@@ -724,7 +724,7 @@ static int vop_plane_atomic_check(struct drm_plane *plane,
 	int max_scale = win->phy->scl ? FRAC_16_16(8, 1) :
 					DRM_PLANE_HELPER_NO_SCALING;
 
-	if (!crtc || !fb)
+	if (!crtc || WARN_ON(!fb))
 		return 0;
 
 	crtc_state = drm_atomic_get_existing_crtc_state(state->state, crtc);